Wednesday, September 13, 2006

War of the Oceans!!!

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

http://www.cdc.noaa.gov/MJO/Predictions/wb2006.pdf

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

Our latest weather-climate discussion dated August 18th, 2006 (and updated September 9th), has been posted on the ESRL/PSD MJO web site at

http://www.cdc.noaa.gov/MJO/Forecasts/climate_discussions.html

Please see past postings for web site links. I am going to discontinue inserting most of them in an effort for brevity. I also need to do the same with these postings.

Global tropical SSTs remain quite warm across the Western Hemisphere and the western/central Indian Ocean, with the greatest positive anomalies around the equatorial date line and the Eastern Pacific cold tongue. Magnitudes in the latter areas were at least plus 2C. In addition, the entire equatorial basin from ~160E to the west coast of South America has observed weekly mean (September 3-9, 2006) positive SST anomalies of ~1-2C. The latest 5-day averaged TAO buoy array data indicates anomalies up to plus 4C around 140W on the equator at 100m depth, with plus 1-2C anomalies common at depths of 100m east of the date line.

Anomalies of roughly plus .5-1.5C were still present across the Atlantic and much of the Indian Ocean. Below normal SSTs ~minus 1-2C continue around Indonesia, particularly south of the equator to the west coast of Australia. A thought is that the combination of cross-equatorial southerly flow linked to Southern Hemisphere cold outbreaks and upwelling has led to these cool anomalies. Hence we see a pattern of generally a “warm Indian Ocean-cool Indonesian region-warm date line region” in regard to SSTAs, typical of a mature warm ENSO event.

During much of the boreal summer, westerly wind anomalies and even actual westerlies have been common particularly along the equatorial date line region, leading to a deepening of the oceanic thermocline along the equatorial cold tongue (discussed above). However, during the last 5-10 days these westerly anomalies have weakened considerably with even weak easterly anomalies. More said about this below.

Actual SSTs in excess of 29C (threshold we use to maintain significant tropical convective forcing) remain present across portions of the Indian Ocean, and especially the equatorial date line and regions of the Caribbean. These SSTs are most extensive from ~150E-180 within 10 degrees of the equator (see links for further details).

Per the latest ENSO diagnostic discussion dated September 13th, 2006, “El Niño conditions have developed and are likely to continue into early 2007” (please see link below). I would agree with that, and only confirms some of the scientific issues discussed in past writings on this Blog. I sincerely applaud CPC for being proactive to get this information out to everyone!!

Nevertheless, I hope to make the point that the magnitude (and duration) of this event as we go into boreal winter, including global impacts, are unclear (understanding other signals such as trends and what statistics offer). There has been a lot of complicated random (stochastic) forcing going on in the coupled atmosphere-ocean system since at least April 2006. Some of these behaviors have been detailed in our latest weather-climate discussion (link above), and are outstanding research issues (let alone using these kind of notions to make predictions). Right now, only careful weather-climate monitoring within the GSDM framework offers any “hope” of catching these kind of complicated highly non-linear dynamical behaviors in real-time.

The following are links to ENSO discussions.

http://www.cdc.noaa.gov/people/klaus.wolter/MEI/

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.html

Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and remember that official USA information on anything related to ENSO comes from CPC.

http://www.cpc.ncep.noaa.gov/products/predictions/90day/

From monitoring, I do feel that the events leading to the onset of weak warm event conditions across the equatorial Pacific Ocean has had at least “episodic impacts” to the global weather during boreal summer. These include general suppression (thus far) of the Tropical North Atlantic tropical cyclone season (understanding climatology and the situation right now), the increase of Tropical Northwest Pacific tropical cyclones (ex., southeast China), the excessive heat much of the USA has dealt with, and the recent change to a cooler and wetter weather pattern for many locations across the Great Plains. The important point to make is there are always forcing-response-feedbacks going on within the Earth-atmosphere-ocean system, and care needs to be offered to thinking that, “some kind of an event is too weak to have an impact”. While there is some truth to these kind of statements statistically (on average), from a weather-climate linkage point of view these notions can lead to difficulty when trying to capture the possibility of extreme weather events. Even on a case by case basis, statistically useful information can be offered for the latter when use of the appropriate framework is employed.

In contrast to the past few months (at least), there are actually at least a couple of weather-climate linkage signals to discuss. The flare-up across the equatorial Indian Ocean (linked to Rossby wave energy dispersions from the North Atlantic) discussed early last week has evolved into a convectively coupled mode at least weakly projecting onto a Madden-Julian Oscillation (MJO). Latest weakly mean OLR anomalies centered ~5-10N/70E were around minus 50-70W/m**2, and a rough calculation gives a phase speed for eastward movement of 5m/s (~4 deg long/day) for this feature (there is also a component moving northeast through India and the Bay of Bengal). The Wheeler phase space plot also lends some support to an MJO. The coherent modes modes Hovmollers show a very weak MJO projection, and also a Kelvin wave emanating to the east. Latest satellite imagery suggests the core of this tropical convective forcing centered ~5-10N/95E, with a Kelvin wave signature moving east along the equator.

There is also a second, less concentrated and weaker area of tropical convective forcing centered along and just north of the date line being forced by the very warm SSTs (discussed above) and other processes. A third (and even weaker) region exists across the Tropical Northeast Pacific. Loosely (for brevity) these last two regions represent an ENSO signal. The forcing across the Tropical North Atlantic and Africa is not well organized.

Now we come to the punch line of this posting that I hope to make clear. We have the convectively coupled dynamical forcing signal over the Indian Ocean and the SST boundary forcing around the date line of the central Pacific Ocean. That is, two areas of tropical forcing, and has been a behavior generally observed since early 2002 (global warming signal?). Which area (ocean) is going to dominate as we go into boreal winter? Obviously there are seasonal cycle considerations and perhaps that may be all we are seeing. Recall that the SSTs are warm across both regions of forcing, with the date line being the warmest where there has been less cloud cover and rainfall recently. Some brief thoughts follow.

Per animations of 150mb and 250mb daily mean vector wind anomalies along with additional tools such as Hovmollers of 250mb meridional wind anomalies, anomalous circulation features have reversed across the subtropical Eastern Hemisphere during the last week. There are now twin anomalous (~10-20m/s, at least) upper tropospheric subtropical anticyclones across the western Indian Ocean and downstream twin cyclones across Indonesia (forced by the divergent outflow) with gyres of the opposite sense in the lower troposphere (leading to some resumption of surface easterly wind anomalies at the date line). This is the expected baroclinic response from a convectively coupled mode such as a MJO.

Impacts onto the extratropics of both hemispheres from the Indian Ocean forcing are currently quite robust. As I type an intense Rossby wave energy dispersion is arcing from this tropical forcing across the North Pacific rim leading to the western USA trough that the numerical models are predicting for the next several days.

Impacts from the date line tropical convection are less robust. Twin subtropical anticyclones continue to appear “off and on”, and have contributed to the subtropical jet (STJ) currently extending into the Desert Southwest. The point is the Indian Ocean tropical forcing is currently dominating the global tropical/subtropical circulation (loosely).

Global AAM signals are still relatively weak and mixed (ex., the mountain torque). There are still regions particularly across the Southern Hemisphere where zonal mean contributions are large particularly from the frictional and possibly the Coriolis torques. Based on the reanalysis data plots through September 10th, global tropospheric relative AAM is ~minus 1 sigma below the 1968-1997 climatology and a bit lower based on the 1979-1998 climatology. The global tendency has become slightly negative with much of that coming from the subtropical atmosphere of both hemispheres and the northern midlatitudes. A time-latitude section of 200mb zonal mean zonal wind anomalies supports this notion, and even shows very weak zonal mean westerly wind anomalies (~2-3m/s) in the equatorial atmosphere. I feel I can link the former with the Indian Ocean forcing and the latter with the date line convection. Speaking for the Asia-North American sector, this is why we are seeing breaking anticyclones across the North Pacific with accelerating westerly flow farther south.

To summarize, I think we have 1) a weak warm ENSO event, 2) a convectively coupled dynamical signal across the Indian Ocean weakly projecting onto a MJO, and 3) a lot of noise including that given by at least 2 areas of tropical convective forcing. There is also an on-going sub-monthly mountain-frictional torque index cycle (not discussed). We also need to think about the role of the seasonal cycle as we transition into boreal autumn, which also adds a huge source of uncertainty. I do think that GSDM Stage 1 best describes the current weather-climate situation.

So, where does the atmosphere go from here? Uncertainty remains as high as it gets. I do think there is some possibility for at least a convectively coupled Kelvin wave to propagate into the western Pacific adding some enhancement to the tropical convection in that region by the end of week 2. It would be most probable for any MJO component to propagate northeast (seasonal cycle) through the Bay of Bengal into the Tropical Northwest Pacific during week 2 and/or week 3. Hence we may observe enhanced tropical convection from the date line to far Southeast Asia by the end of week 2, shifting east week 3 (?). While all this is going on, tropical convection may continue to “hang out” across the equatorial Indian Ocean (SSTs may become at least temporarily to cool to support thunderstorm clusters) perhaps not as intense as what may be occurring across the Northwest Pacific by then. A thought would be to offer GSDM Stage 1 for week 1, transitioning to GSDM Stage 2 for weeks 2-3. Yes, we may see the North Pacific Jet “outrun” the convection anytime (adds more uncertainty).

I also need to add that, on average, the most intense tropical convective forcing could remain across the Indian Ocean during the next few months, which could “affect” any further development of our weak warm ENSO. To me, it will be critical to see how all this tropical forcing possibly becomes more coherent as transition from boreal autumn to winter occurs.

Week 1 (14-20 September 2006): GSDM Stage 1 is most probable. Like nearly all the models show, 1-2 strong mobile synoptic baroclinic troughs digging into the western part of the country then heading east (in a very complicated flow – I cannot talk about “everything” in these postings) is probable. Portions of the Northern Rockies are likely to experience their first heavy snowfall for this upcoming cold season while severe local storms are probable on the Plains. While temperatures change to much colder than normal across the western states, much of the east and Deep South will keep summertime. The Tropical North Atlantic will need to be monitored for any additional cyclone formation. Right now conditions for tropical cyclogenesis there may be the most favorable thus far for 2006. Please see http://www.nhc.noaa.gov/ for the latest tropical cyclone information.

Week 2 (21-27 September 2006): A transition to GSDM Stage 2 may be most probable, meaning an eastward shift of the week 1 pattern across the USA. The trough position may be become established ~95-100W with the ridge just off the west coast into Alaska. Synoptic features would be expected to modulate this pattern.

Week 3 (28-04 October 2006): Persistence of GSDM Stage 2, otherwise unclear.

Once again Southwest Kansas is on the “fence” with a southwest flow storm track. However, there will be a STJ interacting with the strong mobile troughs, and opportunities for some rainfall and storms should exist week 1. The better opportunities will be to our north and east. For weeks 2-3, with a trough axis possibly near or just east of us, we may have a situation of “relatively cool dry northwest flow”. However, individual synoptic systems may shift the trough far enough west to give opportunities of rainfall. Finally, closed lows across the southwest states may also become more probable over the next few weeks interacting with the STJ. This may also be favorable for precipitation across Southwest Kansas.

It is unlikely I will be able to do another posting until about the middle of next week (roughly Tuesday-Wednesday 9/19-20).

Ed Berry

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